External, removable devices may be attached to a personal desktop computer, laptop computer or other processor based system to provide additional functionality to the system. These devices may include, for example, keyboards, mice and other pointing devices, digital cameras, digital music players, storage devices such as disc readers and writers for the Digital Versatile Disc (DVD) format, and external amplified speaker systems, among many others. The Universal Serial Bus (USB) specification describes a bus and a set of protocols for such devices and their interaction with a computer system. Universal Serial Bus Specification Revision 2.0; Universal Serial Bus Specification Revision 1.1, USB Implementers' Forum.
In general, adding an internal or external device such as a removable DVD reader/writer could require a user to install a driver for the device in the operating system of the computer to which it is attached, and may in addition require that the system be shut down to physically connect the device without damage and further that the system be restarted after the installation of the driver, so that the operating system might recognize and enable the device. In contrast, in many instances, a USB compliant computer system such as a personal computer running a Microsoft Windows® XP operating system on an Intel Pentium® 4 processor based platform may automatically load drivers for a USB device in response to the device being connected to the computer system with a standard USB connector and allow immediate use of the device without requiring either a shutdown or a reboot to install the device or its driver.
This is possible in part because the USB Implementers' Forum has defined a set of standard device classes for which the Windows XP and other USB compliant operating systems are preconfigured with drivers. These device classes are described in an additional set of specifications called the device class specifications, such as for one example, the Universal Serial Bus (USB) Device Class Definition for Human Interface Devices (HID), USB Implementers' Forum, which defines a class of devices including, among others, USB keyboards and mice. Whenever a USB device that belongs to a device class that has a class specification approved by the Forum is connected to a USB-compliant computer system, a device driver for the class of devices that is already stored as part of the operating system may be loaded using a standard device class interface for the device class, and allow the use of the USB device. As is well known, class specifications for many different device classes including Mass Storage Devices, Audio devices, Monitor devices, and others are defined by the USB Implementers' Forum.
The near-immediate usability of a USB device newly connected to a running system is further supported by the definition of the USB bus as hot pluggable, that is, the electrical characteristics of the bus and USB devices allow the dynamic connection and disconnection of the devices from a USB hub either on the computer's system board or external to it, without any damage to the computer system or the device in the normal course of operation. Thus, for instance, a USB compliant system that is powered on may be connected to a USB compliant DVD drive and a user may begin to use the drive in conjunction with the system, without any further intervention, after a short delay. Devices that are connected to a hot pluggable bus may also be termed hot pluggable or hot plugging devices.